The invention relates to a bath for electroless depositing tin on metallic and non-metallic substrates, the method making use of this bath and to products produced by means of this method.
Copper layers can be exchanged for thin tin layers, either by means of acid solutions containing thio-urea or derivates, or in solutions containing cyanide. The deposition stops as soon as copper atoms are not visible anymore. Therefore this method is not suitable for an effective protection of copper from atmospheric corrosion. Furthermore, U.S. Pat. No. 3,637,386 discloses electroless tinplating solutions having the V.sup.2+ /V.sup.3+ redox system or the Cr.sup.2+ /Cr.sup.3+ redox system as the reducing agent. These solutions enable the deposition of thicker tin layers. They are, however, very instable, so that they are not very suitable for practical usage. Swiss Patent No. 284,092 discloses a method of tinplating the bearing surface of bearing brasses and bearing bushes. In accordance with this method the relevant surface is contacted with an aqueous, alkaline stannous salt solution for 30-60 minutes at the boiling point, a thin tin layer being applied onto the copper or copper alloy in this manner. Thicker layers (up to 5 .mu.m) are possible at temperatures over 100.degree. C. and by contacting the surface with Al or Zn. This last-mentioned method is very unpractical. Solutions which require a strong alkaline solution at the boiling point for such a long period of time are not very attractive for large scale practical uses. Furthermore, it is a known fact that tin dissolves in boiling alkali hydroxide without cathodic voltage.
So far it has been also assumed that these solutions work on the principle of exchange. The above-mentioned Swiss Patent Specification therefrom mentions only the metallization of copper or copper alloys.